Conventional optical fibers made of quartz, having a capability of excellent optical transmission across a broad wavelength spectrum, have been in practical use mainly for trunk lines. The quartz optical fibers, however, are expensive and poor in workability. Therefore, plastic optical fibers (hereinafter abbreviated as POF), which are less inexpensive, have lighter weight, larger apertures, and easily workable end surfaces, are easily handled, and have other advantages, have been in practical use in a lighting application, a sensor application, and an interior wiring application, such as FA, OA and LAN.
Among various POFs, a step-index (SI) POF having a core/clad structure using polymethyl methacrylate (PMMA) as the core material and a low refractive-index fluorine-containing olefin copolymer as the clad material has gradually been in practical use in the form of a POF cable having coating layers on the outer surface of the POF. Such a POF cable is used as in-vehicle LAN communication wiring from the viewpoint of the capability of high-speed data communication, lighter weight, cost reduction in communication systems, excellent anti-electromagnetic noise measures and the like.
When used in an automobile where the ambient temperature reaches approximately 100 to 125° C., such a POF cable is required to excel in heat resistance. In particular, when installed in a high-temperature environment, for example, in the vicinity of the engine where there are oil, electrolyte, and flammable substances, such as gasoline, the POF cable is required to excel not only in heat resistance but also in chemical resistance. There have therefore been proposed many technologies in which polyamide resins, such as nylon 11, nylon 12 and nylon 612, which excel in heat resistance, chemical resistance and the like, are used as the coating material on the POF cable.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 11-101915), Patent Document 2 (Japanese Patent Laid-Open No. 2001-324626), and Patent Document 3 (Japanese Patent Laid-Open No. 2002-148451) propose POF cables using PMMA as the core material, a vinylidene fluoride (VdF) copolymer having a specific composition providing low crystallizability as the clad material, and nylon 12 resin as the coating material.
Patent Documents 1 and 2 disclose POF cables, the transmission loss of which increases approximately by 5 to 6 dB/km after 1000-hour storage in an environment at 105° C., showing excellent heat resistance in the fixed period. Patent Document 3 discloses a POF cable, the transmission loss of which increases approximately by 7.7 to 26 dB/km after 500-hour storage in an environment at 85° C.
Polyamide resins, such as nylon 12 used as the coating material in the POF cables described in Patent Documents 1 to 3, are industrially obtained through a condensation polymerization reaction between amine and carboxylic acid. However, the produced polymer contains monomers and oligomers derived from the polyamide resin raw material, due to chemical equilibrium. In the configuration in which the POF is in close contact with the coating layer made of a polyamide resin, like the POF cable described in Patent Documents 1 to 3, these monomers and oligomers dissolve and diffuse into the POF in a high-temperature environment, resulting in increase in transmission loss. In particular, when the outermost layer of the clad is made of a fluorine-containing olefin resin and contains a tetrafluoroethylene (TFE) unit as well as at least one of a vinylidene fluoride (VdF) unit and a hexafluoropropylene (HFP) unit, the transmission loss significantly increases.
Examples of the polyamide resin raw material-derived monomers are aliphatic diamino acid compounds, aliphatic dicarboxylic acid compounds, and amino-aliphatic carboxylic acid compounds that form polyamide resins, specifically, 11-aminoundecanoic acid for nylon 11, 12-aminododecanoic acid for nylon 12, hexamethylene diamine and a dodecanedioic acid salt for nylon 612, hexamethylene diamine and a sebacic acid salt for nylon 610, ε-animocaproic acid for nylon 6, hexamethylene diamine and adipic acid for nylon 66, 1,10-decanediamine and 1,12-dodecanediamine for nylon 1010, and 1,12-decanediamine and 1,12-dodecanedioic acid for nylon 1012. Another example is cyclic lactam compounds having an endocyclic amide bond (—CONH—) obtained through intramolecular cyclic esterification of the molecular chain terminals of an aminocarboxylic acid compound. Specific examples are ε-caprolactam for nylon 6 and ω-laurolactam for nylon 12. It is noted that the raw material-derived monomers also include low-molecular-weight compounds produced as by-products during raw material synthesis.
On the other hand, examples of the oligomers include compounds obtained in the course of the condensation polymerization reaction for manufacturing the polyamide resin in which the molecular chain terminals of two or more molecules of the raw material monomers described above (such as aliphatic diamino acid compounds, aliphatic dicarboxylic acid compounds, and amino-aliphatic carboxylic acid compounds described above) undergo intermolecular esterification, so that the molecular chain terminal has functional groups, an amino group (—NH2) and/or a carboxyl group (—COOH); cyclic lactam compounds having an endocyclic amide bond (—CONH—) obtained through further intramolecular esterification of the molecular chain terminals of any of the above compounds; compounds obtained through intermolecular esterification of any of the above compounds; and compounds produced through an intramolecular/intermolecular secondary reaction (deamination reaction or decarboxylation reaction).
When the monomers and oligomers described above are linear, the terminal amino group has high affinity with fluorine-containing olefin copolymers, so that the monomers and oligomers likely tend to stay in the clad layer made of the fluorine-containing olefin copolymer. However, when the monomers and oligomers described above are cyclic lactam compounds, the monomers and oligomers move to the vicinity of the interface to the inner layer side of the clad layer (the core or the first clad layer) to form particulate structures, resulting in increase in structure mismatching at the interface to the POF and hence significant degradation in transmission characteristics of the POF cable.
Among the oligomers described above, those having lower molecular weights more likely tend to dissolve and diffuse into the POF. In particular, those having molecular weights of 2000 or lower significantly show such behavior.
When used in an automobile, a POF cable is required to show small increase in transmission loss for a long period, longer than 5000 hours, in an environment at 105° C. However, the POF cables described in Patent Documents 1 to 3, when the POFs are directly coated with polyamide resin, show increase in transmission loss in a high-temperature environment from the reasons described above, so that the POF cables are not good enough to satisfy the requirements.